9.2 CEBus

The effort for a Consumer Electronics Bus standard was initiated during a Washington, DC, hotel room meeting, sponsored by Electronics Industries Alliance and attended by 12 members representing 12 different companies in April of 1984. Similar to that of X-10, the original goal of CEBus was to develop an infrared remote control standard. Not surprisingly, some parts of the CEBus standards are used for residential and industrial control applications. During 1986, GE's Homenet was selected as the foundation of CEBus protocol. During 1988, an early version of the CEBus Power Line physical layer was proposed using the 1-Kbps Amplitude Shift Keying (ASK) technique. Some techniques for transmission over other media, such as Twisted Pair (TP) and Coaxial Cable (CX), were also proposed thereafter. The current version of the CEBus PL physical layer is based on Intellon's spread spectrum technique proposed during 1991. A control application language was adapted during 1998. CEBus standards were developed to interconnect consumer electronic devices within a home and to link these devices to services provided by external resources at a very economical cost. Details of CEBus standards are described in EIA-600 documents [1 3].

Two components of these CEBus standards have achieved some limited success in real-world applications. The first is the CEBus PL physical layer and the second is the CAL. The CEBus PL physical layer can transmit data packets at about 10 Kbps using a special type of spread spectrum technique. Each CEBus PL packet contains sender and receiver addresses. The CEBus protocol uses a peer-to-peer communications model so that any node on the network has access to the media at any time. To avoid data collisions, it uses a Carrier Sense Multiple Access/Collision Detection and Resolution (CSMA/CDCR) protocol. CAL allows devices to communicate commands and status requests between each other using a common command syntax and vocabulary. CAL defines various electronic device functional subunits as contexts. For example, the audio control of a TV, a stereo, a CD player, or a VCR is a CAL context. Each context is further broken down into objects, which represent various control functions of the context (e.g., volume, bass, treble, or mute functions). Finally, objects are defined by a set of instance variables that specify the operation of the function of the object, such as the default or current setting of the volume object.

9.2.1 Highlights of CEBus Standards

CEBus physical layers are developed for the common in-house transmission infrastructure. There are usually electrical and telephone wiring, and coaxial cable in a typical home environment. These infrastructure wirings usually connect to corresponding external networks over a few nearby entrance points. In addition, Radio Frequency and Infrared wireless means are also considered for exchanging control or multimedia content information. Figure 9.7 shows the CEBus reference architecture based these five different transmission media.

Figure 9.7. CEBus Reference Architecture

A Node 0 near these network entrance points is defined to house external network service distribution devices and routers and/or data bridges to interconnect electronic devices with their CEBus communication means attached to different transmission media. According to the definition of CEBus, a Router in Node 0 is used to relay control information between two wired media. A Data Bridge is used to transfer data or multimedia contents between two wired media and a Brouter is used to interconnect between a wired and a wireless media.

A common CEBus packet format, as shown in Figure 9.8, is defined for use on all different physical layers. A CEBus packet always starts with a preamble for synchronization purposes. The preamble is followed by the DLPDU (Data Link Protocol Data Unit) header, which consists of a control field, a to-address, and a from-address. The 1-byte control field can be used to request one of several acknowledgments. The DLPDU header is followed by the NPDU (Network Protocol Data Unit) header. The NPDU header can be 1 to 8 bytes and is used to send segmentation or routing information. The NPDU header is followed by the APDU (Application Protocol Data Unit) header. The APDU can be either 1 byte for nonsecure service requests or multiple bytes for security authentication services. The APDU header is followed by a CAL message of variable length. The CEBus packet ends with an FCS (Frame Check Sequence) of 8 bits. This packet format reflects CEBus's protocol of five layers: CAL, application, network, data link, and physical. The preamble field is terminated with a PEOF (Preamble End Of Field) symbol. All other fields are separated by an EOF (End Of Field) symbol and a CEBus packet is terminated by an EOP (End Of Packet) symbol.

Figure 9.8. CEBus Packet Format

One and zero as well as some symbols with special meanings are all coded by their durations of signaling as shown in Table 9.2. A CEBus UST (Unit Symbol Time) is defined as 100 µs. A 1 bit consumes 1 UST and a 0 bit consumes 2 UST. Adjacent symbols and their durations are distinguished by alternating Superio and Inferio states of signaling. Depending on a particular physical medium, Superio and Inferio states can be represented with quite different methods. For the CEBus PL physical layer, the Superio state is represented by the normal phase of a spread spectrum burst, and the Inferio state is represented by the opposite phase of the spread spectrum burst except for the preamble period. For the CEBus CX physical layer, the Superio state is represented by the presence of a 5.5-MHz carrier, for control information, and the Inferio state is represented by the absence of the carrier.

Table 9.2. CEBus Symbol Format

Symbol	Duration (µs)	Length (UST)
One	100	1
Zero	200	2
EOF	300	3
PEOF	800	8
EOP	400	4

9.2.2 CEBus PL Physical Layer

A chirp is a CEBus PL physical layer base signaling unit. The spread spectrum characteristics of the CEBus PL physical layer is the result of the special waveform of its chirp as shown in Figure 9.9. To achieve the maximum autocorrelation gain in a CEBus receiver, the chirp waveform should be constructed exactly 360 digitized points according to the CEBus standards [2]. A chirp waveform starts at 200 kHz and sweeps to 400 kHz, jumps to 100 kHz, and then sweeps to 200 kHz. The complete waveform takes 25 cycles in 100 µs. The chirp waveform is limited to a peak-to-peak voltage of 7 V while the out-of-band voltage should be less than 5 mV at below 100 kHz and below 1 mV above 400 kHz. This can be achieved with the help of additional bandpass filters. With a load impedance of 10 ohms, the in-band PSD level of the CEBus PL physical layer signal is about 27 dBm/Hz as shown in Figure 9.10.

Figure 9.9. CEBus PL Spread Spectrum Chirp

Figure 9.10. PSD of CEBus PL Signal

The basic chirp waveform defined in the CEBus standards is used as the Superio state. The Inferio state is represented by the absence of the waveform during preamble only and by the phase inverse of the waveform in the rest of the packet. In other words, a 0 bit is represented by the absence of the chirp waveform for 200 µs during the preamble and by two inverted chirp waveforms in the following fields. EOF and EOP can take either Superio or Inferio waveforms depending on if the preceding last bit is 0 or 1, respectively.

Similar to that of X-10, a complete CEBus PL transceiver implementation requires signal and packet generation and reception parts. Because of its higher transmission throughputs and signal level, a CEBus PL transceiver is normally implemented with an Application-Specific Integrated Circuit (ASIC) chip set such as these P300 and P111 chips from Intellon and the CEWay PL-One from Domosys. Figure 9.11 shows the application of Intellon chips for a CEBus transceiver.

Figure 9.11. A CEBus PL Implementation Example

A typical CEBus transceiver can be constructed with a transceiver chip and a power amplifier in conjunction with a host device microprocessor as well as some other discrete components. The transceiver chip is responsible for resource-intensive data link functions and physical layer services of the protocol. Specific DLL services include transmission and reception of packets, byte-to-symbol conversion for transmitted packets, symbol-to-byte conversion for received packets, transmit channel access (based on EIA-600 access rules), and CRC generation and checking. The power amplifier and associated discrete components amplify the transmitted signal to drive the low impedance of the medium, couple the spread spectrum signal onto the medium, and filter the incoming signal. The host communicates with the SSC P300 by issuing commands. These commands provide for the initialization and verification of the node's operating mode and addresses, for the transmission and reception of packets, and for the return of status information.

9.2.3 CEBus CX Physical Layer

The use of residential dual coaxial cable for exchanging control and data information is also defined by the CEBus standards [3], but they are not broadly recognized. The structure of Node 0 for the CEBus CX physical layer is worth a close examination because any other residential coaxial cable based transmission system will face the same signal redistribution problem at a cable TV entrance point. Figure 9.12 shows the coaxial cable portion of the Node 0 structure defined by the CEBus.

Figure 9.12. Central Connection Diagram

The dark external coaxial cable is connected to the external cable or off-the-air TV signal over a splitter. Parallel with the external cable, an internal coaxial cable, in gray, is connected to another nearby splitter. About 40 dB of signal loss occurs between different output ports of a splitter. To distribute the control or data information from a particular coaxial cable outlet to the other devices connected through the splitter, a feedback mechanism is used at the input port of the same splitter. As recommended by the CEBus standards, the feedback mechanism consists of a two-port splitter, a bandpass filter, and a block converter. Control or data information from a particular transmitter is collected by the bandpass filter, converted to another frequency band, and sent to the rest of the receivers through the same splitter. Because transmit and received signals operate in different frequency bands, some signal amplification can also be introduced in the block converter to compensate attenuations caused by cable and splitters.

Because of the use of a block converter, the CEBus CX control signal can be transmitted with a carrier frequency of 5.5 MHz and received at 4.5 MHz as shown in Figure 9.13. Similarly, data information can be transmitted in the frequency band of 54 to 150 MHz on the internal coaxial cable and received in the 324- to 420-MHz band on the same coaxial cable. At a carrier frequency of 4.5/5.5 MHz, the Superio state is represented by the presence of the carrier, and the Inferio state, by the absence.

Figure 9.13. An Example Frequency Allocation on Internal Coaxial Cable

The same redistribution problem can also be addressed with a simpler solution as shown in Figure 9.14. We can leave the input port of a splitter open to let the redistribution be taken care of by the natural reflection of the open port. Because of the strong reflection, the external cable or off-the-air TV signal can be connected to another output port of the splitter for distribution at its original frequency.

Figure 9.14. Redistribution on a Single Coaxial Cable